A 33-year-old woman has a transient ischemic attack. She takes no medicines and is not pregnant, and blood was drawn before any anticoagulant was given. Results: RPR reactive at 1:4 (also reactive at 1:4 when she donated blood 8 months ago); TP-PA nonreactive; PT 12.0 s (reference 11.0-13.5 s); aPTT 47 s (reference 25-35 s), immediate 1:1 mix with normal plasma 45 s; dRVVT screen/confirm normalized ratio 1.9 (positive above 1.2); anticardiolipin IgG 68 GPL units (positive above 40); platelets 118 x 10^3/µL [118 x 10^9/L]. Which single explanation best accounts for all of these results?

Latent syphilis, TP-PA falsely negative

Heparin contamination of the sample

A 9-year-old with chronic diarrhea and poor growth has a negative tissue transglutaminase (tTG) IgA. Total IgA is below 5 mg/dL [0.05 g/L]. Which test should be done next?

Deamidated gliadin peptide (DGP) IgG

A 7-year-old has dark urine, edema and a low C3 three weeks after impetigo. ASO is 150 IU/mL (reference < 200). Which test best supports a recent group A streptococcal infection?

Throat culture for group A streptococci

Test result interpretation, confirmatory testing and disease correlation

9 ASCP MLS practice questions on Test result interpretation, confirmatory testing and disease correlation, part of Immunology. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 33-year-old woman has a transient ischemic attack. She takes no medicines and is not pregnant, and blood was drawn before any anticoagulant was given. Results: RPR reactive at 1:4 (also reactive at 1:4 when she donated blood 8 months ago); TP-PA nonreactive; PT 12.0 s (reference 11.0-13.5 s); aPTT 47 s (reference 25-35 s), immediate 1:1 mix with normal plasma 45 s; dRVVT screen/confirm normalized ratio 1.9 (positive above 1.2); anticardiolipin IgG 68 GPL units (positive above 40); platelets 118 x 10^3/µL [118 x 10^9/L]. Which single explanation best accounts for all of these results?
1. AAcquired factor VIII inhibitor
2. BLatent syphilis, TP-PA falsely negative
3. CHeparin contamination of the sample
4. DAntiphospholipid antibodies
Show answerHide answer
Answer: D · Antiphospholipid antibodies
The RPR uses a cardiolipin-lecithin-cholesterol antigen, and antiphospholipid antibodies can react with it, giving a biologic false-positive result; the nonreactive TP-PA shows no treponemal antibody. A reactive RPR lasting more than 6 months, as here, points to a chronic cause such as antiphospholipid syndrome or SLE. The same antibodies act as a lupus anticoagulant: they prolong the aPTT, the 1:1 mix does not correct it, and the dRVVT ratio is positive. With a positive anticardiolipin IgG, mild thrombocytopenia and an arterial event in a young woman, the results fit antiphospholipid syndrome, and the antibodies must be confirmed at least 12 weeks later. A factor VIII inhibitor causes bleeding rather than thrombosis, and neither it nor heparin makes the RPR reactive.
- A: A factor VIII inhibitor causes bleeding, does not prolong the dRVVT and does not make the RPR reactive.
- B: TP-PA is very sensitive in latent syphilis, so a nonreactive result makes syphilis very unlikely.
- C: No heparin was given, and heparin would not make the RPR reactive.
Question 2 · Hard
A 9-year-old with chronic diarrhea and poor growth has a negative tissue transglutaminase (tTG) IgA. Total IgA is below 5 mg/dL [0.05 g/L]. Which test should be done next?
1. ADeamidated gliadin peptide (DGP) IgG
2. BNative gliadin IgA antibody
3. CRepeat tTG IgA in 6 months
4. DEndomysial antibody (EMA) IgA
Show answerHide answer
Answer: A · Deamidated gliadin peptide (DGP) IgG
Celiac IgA tests are falsely negative in selective IgA deficiency, which is about 10 times more common in celiac disease than in the general population. Total IgA should be measured with the tTG IgA, and when it is very low, IgG-based tests such as DGP IgG or tTG IgG are used. Endomysial and gliadin IgA antibodies fail for the same reason. The diagnosis is usually confirmed by small-bowel biopsy while the patient still eats gluten.
- B: Native gliadin IgA is less accurate and also needs IgA.
- C: A repeat IgA test cannot turn positive in a patient without IgA.
- D: EMA IgA is also an IgA test and will be falsely negative.
Question 3 · Hard
A 7-year-old has dark urine, edema and a low C3 three weeks after impetigo. ASO is 150 IU/mL (reference < 200). Which test best supports a recent group A streptococcal infection?
1. AHeterophile antibody test
2. BThroat culture for group A streptococci
3. CRepeat ASO on a fresh sample
4. DAnti-DNase B
Show answerHide answer
Answer: D · Anti-DNase B
Post-streptococcal glomerulonephritis after a skin infection often shows a normal ASO, because lipids in the skin inactivate streptolysin O and the antibody response to it is weak. Anti-DNase B rises after both throat and skin infections and stays high for months, so it is the better marker here. Testing both antibodies detects more recent infections than either alone. A low C3 with hematuria and edema fits the diagnosis.
- A: Heterophile antibodies point to EBV infectious mononucleosis.
- B: The infection was in the skin, and cultures are usually negative by the time nephritis appears.
- C: A repeat ASO will likely stay low after a skin infection.
Question 4 · Medium
Acute and convalescent sera, collected 2 weeks apart, are titrated in the same run. Which pair shows a significant rise in antibody?
1. A1:16, then 1:32
2. B1:64, then 1:64
3. C1:8, then 1:64
4. D1:128, then 1:64
Show answerHide answer
Answer: C · 1:8, then 1:64
A rise of at least four times (two doubling dilutions) between acute and convalescent sera is the usual sign of recent infection. A rise from 1:8 to 1:64 is eightfold. A single doubling, from 1:16 to 1:32, is within the error of the method. The two samples should be tested in the same run, because between-run variation can be one dilution.
- A: A twofold rise is within the error of a doubling-dilution test.
- B: An unchanged titer suggests past infection or immunity.
- D: A falling titer suggests a resolving infection, not a new one.
Question 5 · Easy
A screening test, used to find as many cases as possible, should above all have high:
1. ASensitivity
2. BSpecificity
3. CPositive predictive value
4. DReportable range
Show answerHide answer
Answer: A · Sensitivity
Sensitivity is the share of people with the disease who test positive, so a highly sensitive test misses few cases and a negative result helps rule disease out. Screening tests are chosen for this reason. The price is more false positives, so positive screens go on to a confirmatory test with high specificity. Many serologic algorithms, such as those for syphilis and hepatitis C, work this way.
- B: High specificity is the main goal of the confirmatory test.
- C: Positive predictive value depends on prevalence and is often low for screens in low-risk groups.
- D: Reportable range concerns quantitative results, not case finding.
Question 6 · Medium
A moderately high anticardiolipin IgG is found once in a patient being evaluated for antiphospholipid syndrome. Before the result counts toward the criteria, it must be:
1. APositive again within 7 days
2. BSupported by a positive anti-dsDNA
3. CConfirmed by a reactive RPR
4. DPositive again at least 12 weeks later
Answer and explanation in LearnFree account
Question 7 · Hard
A woman at 12 weeks of pregnancy has positive Toxoplasma IgG and IgM. Her IgG avidity is high, and the kit states that high avidity excludes infection in the previous 4 months. The best interpretation is:
1. AThe IgG result is a false positive
2. BPrimary infection occurred in the last month
3. CInfection was acquired before this pregnancy
4. DThe fetus is very likely infected
Answer and explanation in LearnFree account
Question 8 · Hard
A screening immunoassay has 99% sensitivity and 98% specificity. It is used in a population of 10,000 in which the prevalence is 1%. What is the positive predictive value?
1. A98%
2. B99%
3. C33%
4. D67%
Answer and explanation in LearnFree account
Question 9 · Medium
Which finding in a newborn's serum best shows that the infant, not the mother, made the antibody?
1. ASpecific IgG at the mother's titer
2. BSpecific IgM antibody
3. CTotal IgG above the adult range
4. DSpecific IgG that is gone by 12 months
Answer and explanation in LearnFree account

Test result interpretation, confirmatory testing and disease correlation

A 9-year-old with chronic diarrhea and poor growth has a negative tissue transglutaminase (tTG) IgA. Total IgA is below 5 mg/dL [0.05 g/L]. Which test should be done next?

A 7-year-old has dark urine, edema and a low C3 three weeks after impetigo. ASO is 150 IU/mL (reference < 200). Which test best supports a recent group A streptococcal infection?

Acute and convalescent sera, collected 2 weeks apart, are titrated in the same run. Which pair shows a significant rise in antibody?

A screening test, used to find as many cases as possible, should above all have high:

A moderately high anticardiolipin IgG is found once in a patient being evaluated for antiphospholipid syndrome. Before the result counts toward the criteria, it must be:

A woman at 12 weeks of pregnancy has positive Toxoplasma IgG and IgM. Her IgG avidity is high, and the kit states that high avidity excludes infection in the previous 4 months. The best interpretation is:

A screening immunoassay has 99% sensitivity and 98% specificity. It is used in a population of 10,000 in which the prevalence is 1%. What is the positive predictive value?

Which finding in a newborn's serum best shows that the infant, not the mother, made the antibody?